Add #headers to Savon::Response

Shout out to @ashrocket for suggesting this. Closes #544.

lib/savon/response.rb

@@ -16,6 +16,10 @@ def body
     end
     alias to_hash body
 
+    def header
+      hash[:envelope][:header]
+    end
+
     def hash
       @hash ||= nori.parse(raw)
     end

spec/savon/response_spec.rb

@@ -0,0 +1,17 @@
+require 'spec_helper'
+
+describe Savon::Response do
+  subject(:response) do
+    response = Savon::Response.new(nil)
+    response.send(:instance_variable_set, :@hash, {
+      :envelope => {:header => 'the-spice-must-flow'}
+    })
+    response
+  end
+
+  describe '#headers' do
+    it 'returns the headers for the underlying response' do
+      expect(response.header).to eq('the-spice-must-flow')
+    end
+  end
+end